1. Field of the Invention
The present invention relates generally to optical apparatus for focusing a light source, and more particularly to such apparatus using optical elements including holographic optical elements to focus a polychromatic source to an extended focal pencil.
2. Brief Description of the Prior Art
In optical communications, data storage, sensing and imaging apparatus and the like, laser devices are used as a light source, and optical means are provided to focus the laser beam to a small spot. In these applications laser diodes are normally the preferred form of laser, but because they are subject to astigmatism and beam divergence, they typically require bulky and expensive correction optics. A need thus exists for inexpensive, easily reproducible and compact correction optics. There are also a variety of optical manufacturing applications (lithography, laser welding/cutting, surgery, etc.) in which extended focal depth is a fundamental requirement for uniformity of the operation. For example, in laser cutting, a diverging beam leads to an undesirable variation of the cutting thickness with depth. Furthermore, in many of the applications mentioned above, the apparatus is quite sensitive to position of the focusing optics relative to the surface or object upon which the beam is to be directed, and as a result an expensive servo-mechanism must be used to continuously adjust the focus to the position of the surface. A need thus exists for an optical means having an extended focal depth such that the apparatus is relatively insensitive to minor changes in position of the work surface illuminated.